That's a very difficult question you're asking, especially for native English speakers. They were born and raised in English-speaking countries and thinking in English is the last thing they have to think about.
It all depends on what English level you want to attain, how much enthusiasm you have and how much you interact with native speakers. I know they're not readily available unless you live in English spoken countries.
Unfortunately it would be that much harder without the interaction, because no matter how much authority you give to whatever English studying material, your brain would not. It will finally admit what you've learned through studying materials are correct once you interact with native English speakers.
I won't lie to you, it will take years even with all the right environment and support. This is my 7th year in Canada and my English code is slow and half-assed.